1. Field of the Invention
The present invention relates to a network system including a plurality of nodes which are connected through physical or logical connections, and in particular to the arrangement of each node in the network system wherein a control signal is transferred from a sender node to a receiver node through at least one intermediate node.
2. Description of the Related Art
In a digital ring system where a plurality of nodes are connected through transmission cables in a ring configuration, several methods have been proposed for protecting network services against failure. One of the protection methods, a self-healing technique is known which provides traffic protection by switching a working line to a protection line when failures that affect the working line occur. Recently attention has been given to the self-healing technique of the Bidirectional Line-Switched Ring (BLSR) system, wherein the Automatic Protection Switching bytes (hereinafter referred to as APS bytes) are used to implement the BLSR self-healing method. Such a method is disclosed in "SONET (Synchronous Optical Network) Bidirectional Line-Switched Ring Equipment Generic Criteria" (GR-1230-CORE Issue 1, December 1993).
More specifically, consider a 4-fiber BLSR system as shown in FIG. 1, wherein nodes N1-N4 are connected in a ring configuration through working lines BL.sub.1 -BL.sub.4 and protection lines BL.sub.11 -BL.sub.14. When detecting failures that affect the working line BL.sub.4 and the protection line BL.sub.14, the node N1 generates a switch request using the APS bytes and transmits it to the destination node N4 which is connected to the node N1 through the working line BL.sub.4 and the protection line BL.sub.14. The APS bytes for the switch request are transferred to the node N4 through the working lines BL.sub.1 -BL.sub.3 while passing through the nodes N2 and N3 which are not its final destination. When the APS bytes reach final destination node N4, the node N4 performs the appropriate switch action according to the APS bytes received from the source node N1 and a confirmation is sent back to the source node N1. In this manner, the BLSR protection switching is completed after the source node N1 receives the confirmation from the destination node N4.
Referring to FIGS. 2A and 2B, in the SONET overhead, the APS bytes consist of a K1 byte and a K2 byte which are used to coordinate switching activity. The last four bits of the K1 byte are used for the node ID of the destination and the first four bits of the K2 byte are used for the node ID of the source. The first four bits of the K1 byte and the last four bits of the K2 byte are used for control data. The more detailed description is provided in the above document (6-13 to 6-17, GR-1230-CORE Issue 1, December 1993).
As shown in FIG. 3, each node is comprised of a receiver 11, a processing unit including an APS-byte analysis processor 12, and a transmitter 13. The APS-byte analysis processor 12 analyzes the APS bytes received from one adjacent node and performs an appropriate activity such as the protection switching or enters the pass-through state according to the received APS bytes. More specifically, when the destination ID of the received APS bytes is not identical to the node's own ID, the APS-byte analysis processor 12 transfers the received APS bytes, as is, to the next adjacent node. This pass-through state is referred to as a full pass-through state and the node in this state is referred to as an intermediate node. On the other hand, if the received APS bytes are at its destination node, that is, the destination ID of the received APS bytes is identical to the node's own ID, the APS-byte analysis processor 12 performs the appropriate switching and transmits new APS bytes indicating the acknowledgment of the switch request back to the source node. In FIG. 1, the nodes N2 and N3 are intermediate nodes and the nodes N1 and N4 complete the protection switching when the source node N1 receives the new APS bytes from the destination node N4.
In the above conventional transferring system, however, the APS-byte analysis processor 12 of each node located between the source and destination nodes included performs the APS-byte analysis and processing each time APS bytes are received. In general, the processing time T.sub.PRO required for the APS-byte analysis and processing is of the order of several milliseconds, which is obtained by the equation: T.sub.PRO =T.sub.DBT +T.sub.GEN +T.sub.OUT, where T.sub.DET is the time required for detection of the APS bytes, T.sub.GEN is the time required for generation of new APS bytes, and T.sub.OOT is the time required for output processing of the new APS bytes.
Therefore, rapid protection switching cannot be achieved without providing each node with the very high-speed processing unit. In general, the elapsed time from failure detection to protection switching completion can be obtained as the sum of the time required for switching at both source and destination nodes and the transmission delay time. Since the transmission delay time is a function of the number of nodes, the larger the number of nodes in the ring system, the longer the elapsed time until protection switching completion. Therefore, according to the conventional system, the high-speed protection switching cannot be achieved in the ring system, especially, one which includes a large number of nodes.
Alternatively, it is considered that the entire APS processor is separately formed with hardware so as to increase in processing speed. However, such a system results in the complicated circuit of the processing unit and an increased amount of hardware.